I've tested this locally and it works like a charm.
|mediawiki/extensions/InputBox||master||+4 -2||Adding CSS class to hook into SearchSuggest feature.|
@freephile: See https://www.mediawiki.org/wiki/Gerrit/Commit_message_guidelines how to make gerritbot realize. :)
Does that mean https://gerrit.wikimedia.org/r/c/mediawiki/extensions/InputBox/+/620370/ should be abandoned? For future reference, feel free to amend instead.
Belated comment: this makes zero sense for most of the uses of such search forms, which typically concern searching on a specific subset of pages and not everywhere on a wiki. Search suggestions just come up meaningless in most cases, especially in places like https://en.wiktionary.org/wiki/Wiktionary:All_Thesaurus_pages where the target of search box and search suggestions lead people to different places. This was a bad patch to merge essentially without discussion.
(If it makes sense for individual search forms, there should be an option to specify the class to input for authors instead.)